Resistance material and method for making the same



Dec. 15, 1970 NQBUMASA QSHI MA ET AL I 3,547,834

RESISTANCE MATERIAL AND METHOD FOR MAKING THE SAME Filed Sept. 5, 1967 5 Shoots-$110M; z

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Dec. 15, 1970 I NOBUMASA os ET AL 3,547,834

RESISTANCE MATERIAL AND METHOD, FOR MAKING THE SAME Filed Sept. 5, 1967 3 Sheets-Sheet 3 INVENTORS Mosumnn (NW/HR,

yaw/a swam A/DRIHIRU Tq V/ United States Patent Office 3,547,834 RESISTANCE MATERIAL AND METHOD FOR MAKING THE SAME Nobumasa Oshima and Yoshio Enoki, Hirakata-shi, and

Norihiro Tani, Moriguchi-shi, Japan, assignors to Matsushita Electric Industrial Co., Ltd., Osaka, Japan, a corporation of Japan Continuation-impart of application Ser. No. 394,832, Sept. 8, 1964. This application Sept. 5, 1967, Ser. No. 665,515

Int. Cl. B44d 1/20; H01b 1/06 US. Cl. 252511 Claims ABSTRACT OF THE DISCLOSURE There is disclosed carbonaceous minute particles characterized by high electric resistivity, low temperature dependence of the resistivity and other excellent properties, to be used for making the composition type resistors, and also a method to make such material.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part application of the pending United States patent application No. 394,- 832 filed on Sept. 8, 1964, now abandoned.

This invention relates to a resistance material in a powder form adapted for composition type resistors comprising carbonaceous minute particles coated, at least partially at the surface thereof, with a thin film in a non-crystalline form consisting essentially of silicon, oxygen, carbon and hydrogen atoms and to a method for making the same.

Conventional carbon composition type resistors in a body type or in a film type have been manufactured by employing carbonaceous minute particles dispersed in a polymerizable resin. The carbonaceous minute particles having a relatively high electric conductivity are suitable for making a carbon composition type resistor having a low electrical resistance. However, it has been ditficult for such carbonaceous minute particles to form a composition type resistor having a relatively high electric resistance combined with superior electrical characteristics and a high production yield.

One method for making a composition type resistor having a relatively high electric resistance is to reduce a weight ratio of the carbonaceous minute particles to a polymerizable resin. However, such method has a disadvantage that so produced resistors are inferior in the voltage coefficient of resistance and the noise factor and have a large tolerance in the resultant resistance because the lower ratio of the particles to polymerizable resin results in a decrease in the number of the electrical contacts in the unit volume. Another method is to employ carbonaceous minute particles having a high electric resistivity and can eliminate said disadvantage resulted from the first method. However, there have been known heretofore no carbonaceous minute particles having a high electric resistivity and a low temperature dependence of the electric resistivity because conventional carbonaceous minute particles are increased in the electric resistivity while being increased in the temperature dependence of the electric resistivity. For example, minute particles of a carbonaceous material obtained by the thermal decomposition of silicone resin are known to have electric resistivity higher than that of conventional carbonaceous minute particles but are provided with a high temperature dependence of the electric resistivity. In addition, such carbonaceous minute particles differ in the bulk density, water absorption, particle shape and particle size from the conventional carbonaceous minute particles and 3,547,834 Patented Dec. 15, 1970 are not suitable for use in a material of carbon composition type resistors.

The last method is to provide conductive particles such as carbon or metal particles coated at least partially With an insulating material such as polymerized resin and to mix said precoated conductive particles with a binding material. Such method, however, increases the number of the particles having no electrical conductivity and decreases the number of the electrical contacts in the unit volume by coating the surfaces of the conductive particles with an electrical non-conductive material.

SUMMARY OF THE INVENTION An object ofthe invention is to provide carbonaceous minute particles characterized by a high electric resistivity and a low temperature dependence of the electric resistivity.

Another object of the invention is to provide a method for making a resistance material characterized by a high electric resistivity and a low temperature dependence of the electric resistivity.

The last object of the invention is to provide a method for making a resistance element characterized by a relatively high electric resistance, a low temperature dependence of resistance, a superior noise factor and a low voltage coefficient of resistance with high production yield.

These objects of the invention will be apparent upon consideration of the following description taken together with the accompanying drawings.

FIG. 1 is a graph illustrating a relation between Sicontent in the resultant resistance material according to the present invention and the mixing ratio of silicone resin to carbon black as a function of carbonizing temperature of said silicone resin.

FIG. 2 is a graph illustrating a relation between the electric resistivity of resultant resistance material according to the present invention and the carbonizing temperature of the silicone resin as a function of a Si-content.

FIG. 3 is a graph illustrating a relation between the electric resistance of a resistance element comprising a resistance material according to the present invention and the carbonizing temperature of the silicon resin as a func: tion of a Si-content in said resistance material.

FIG. 4 is a graph illustrating a temperature dependence of electric resistance of a resultant resistance element comprising a resistance material according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It hasbeen discovered according to the present invention that a resistance material in a powder form comprising carbonaceous minute particles coated, at least partially at the surface thereof, with a carbonized silicone resin can be prepared by manufacturing steps, in combination, mixing carbonaceous minute particles in a solvent solution of thermosetting silicone resin, evaporating a solvent of said solution, heating the resultant mixture to cure said silicone resin, p'ulverizing the heated mixture and firing the thus treated mixture at a temperature of 700 C. to l,300 C. in a none-oxidizing atmosphere.

A cured silicone resin is decomposed to a carbonized silicone resin losing volatile or tarry decomposition products by firing in a non-oxidizing atmosphere at a temperature about 250 C. to 1000" C. Said carbonized silicone resin exists in an amorphous state by X-ray analysis and mainly comprises Si (about 10 to 55 wt. percent), C (about 10 to wt. percent) and 0 (about 5 to 35 wt, percent) atoms in addition to minor H atoms (0 to 3%). When the carbonizing temperature is sufficiently W W W0 wherein W is a weight of a drysample and W is a weight of a sample which is kept at 25 C. for 24 hours in a relative humidity of 80%.

TABLE 1 Electric Vapor resistivity 1 absorption Carbonizlng temperature C.) (em.) (wt. percent) (4. 5X10) (0. 5) 1. 6x10 9. 6 6. 6x10 10. 1 3. 2X10 0.3 1. 1X10 O. l

TABLE 2 Hall coeliicient (X cmfi/coulomb) Temperature C.) 150 0 -l75 (A) Oarbonized silicone resin 1 5. 6 -7. 0 -9. 6 (B) Pure carbonaceous minute particles 1 +0. 356 +0. 204 +0. 047 (C) Mixture ofA and B +0. 455 +0. 315 -0. 037 (D) The present invention +2. 01 +2. 38 +2. 72

l KR282 (the same silicone resin as used in Example 2) carbonized at 1,000 O. for 1 hour in vacuo. This sample is in a brick form.

2 Excelsior (the same carbon black as used in Example 2) heattreated at 1,200 C. for 1 hour in vacuo.

3 The carbonized silicone resin powder used in Example 2 and the pure faibonaeeous minute particles defined at 2 are mixed in the weight ratio This sample ls made in a similar way to the process described in Example 2 applying the carbon black defined at 2.

Said carbonaceous minute particles can be made of any of carbon powder such as, for example, channel black, furnace black, thermal black, acetylene black, lamp black, natural graphite or artificial graphite powder. Such carbon powder is preferably preheated in a non-oxidizing atmosphere at a temperature from 700 C. to 2500 C.

An operable average particles size of said carbon powder ranges from about 8 mp. to 1011-. A resistance material which makes a resistor superior in the voltage dependence of electric resistance and noise factor can be prepared by employing carbon powder having an average particle size of about 8 to 60 mg. The average particle size referred to herein is determined by an electron microscopic observation of about 100 particles.

Silicone resin used in this invention are thermosetting organopolysiloxanes which form three dimensional net works by curing, and are supplied commercially as varnish which contains partially polymerized organosiloxane, sol vent such as toluene, xylene and gasoline and a catalyzer such as Zn, Pb and Co salts of fatty acids. Silicone oils and silicone rubbers can not be used. Either of common silicone resins such as alkyl-silicones, aryl-silicones, alkylaryl-silicones or same silicone resins modified with organic plastics may be used. Specially included in this category are methyl, ethyl, propyl, butyl, amyl, phenyl, naphthyl, benzyl silicones alone or co-condensed with one another 4 or same silicones modified with organic plastics such as, for example, alkyd, phenol, polyester, melamine and epoxy resin.

A Si-content in the starting silicone resin relates to a Si-content in the carbonized silicone resin and has a great effect on the electric resistivity of the resultant resistance material according to the present invention. For instance, a silicone resin which is rich in methyl silicone ingredient and which has relatively high Si-content, gives a higher electric resistivity than a silicone resin which is rich in phenyl silicone ingredient and which has a relatively low Si-content. A silicone resin modified with organic plastics which has the lowest Si-content gives the lowest electric resistivity.

It is important that a weight ratio of aforesaid thermosetting silicone resin to said carbon powder is from 0.02 to 1.

One can use any solvent which dissolves silicone varnish. It is desirable that a solvent has a boiling point not higher than C. Operable solvents are benzene, toluene, xylene, methanol, ethanol, propanol, butanol, acetone, trichloroethylene and gasoline.

The carbon powder is mixed with said silicone varnish dissolved in a suitable amount of said solvent and if necessary, with a dispersing agent such as stearic acid or surface active agents by any conventional method, for example, by using a ball mill or a mixer. The solvent contained in the mixture is removed by heating at a temperature higher than the boiling point thereof. The silicone resin is cured by heating the mixture at a curing temperature of the resin employed. The curing mixture is then pulverized to fine powder having an average grain size smaller than 10;. The pulverizing step may be carried out after the carbonizing (or firing) step. S0 produced fine powder is composed of the carbon particles coated, at least partially at the surface thereof, with the cured silicone resin. Said fine powder is fired in a non-oxidizing atmosphere, that is, in vacuo or in an inert gas such as argon or nitrogen or a reducing gas such as hydrogen or carbon monoxide so as to carbonize the surface layer of cured silicone resin. The resultant material is composed of the carbon particles coated with thin surface layers of carbonized silicone resin.

It has been discovered according to the present invention that the carbonizing temperature of 700 to 1300 C. produces a resistance material having a high electric resistivity and a low temperature dependence of the electric resistivity. A carbonizing temperature below 700 C. results in an extremely high electric resistivity of said thin layers of carbonized silicone resin which coats the carbon particles at the surfaces thereof as shown in Table 1, and does not produce the aimed effects of this invention. In contrary, a carbonizing temperature over 1300 C., results in a relatively low electric resistivity of the resultant material. The carbonizing temperature of 700 to 1000 C. produces a resistance material characterized by a higher electric resistivity, while the carbonizing temperature of 1000 to 1300 C. produces a resistance material superior in the water absorption in accordance with the present invention.

The electric resistivity of the novel resistance material can be controlled by selecting the starting carbon powder. The resultant electric resistance material according to the present invention is provided with a relatively high electric resistivity by employing channel black as a starting carbon powder and is provided with a relatively low electric resistivity by employing acetylene black or conductive furnace black as a starting carbon powder.

A Si-content in the resultant resistance material has also a great effect on the electric resistivity. Said Si-content can be obtained by selecting a kind of the starting silicone resin or by adjusting a mixing ratio of the starting carbon powder to the starting silicone resin. The resultant resistance material comprises silicon, carbon, oxygen and hydrogen atoms in a weight percent shown in Table 3.

Operable weight ratio of silicon atoms to total carbon atoms in the resultant resistance material are ranges from about 0.005 to 0.3, preferably from 0.005 to 0.125 in accordance with the invention.

The temperature dependence of electric resistivity of the resultant resistance material can be controlled by preheating the starting carbon powder and by selecting the carbonizing temperature. The resistance material can be improved in the temperature dependence of resistivity by preheating the starting carbon powder, especially channel black or lamp black in a non-oxidizing atmosphere such as vacuum, nitrogen or hydrogen at a temperature of 700 to 2500 C. in advance to the mixing step.

In the process of making the novel resistance material according to the present invention, a higher carbonizing temperature results in a lower temperature dependence of electric resistivity of the resultant resistance material.

The resistance material of the present invention can form resistance elements for use in making composition resistors by any suitable and conventional method. For instance, the resistance material of the present invention is mixed with polymerizable resin and, if necessary, with filler and solvent. The mixture is applied, in a thin film, to a non-conductive base or molded to a given shape, and then the polymerizable resin in the mixture is cured so as to produce a composition resistance element in film type or body type.

EXAMPLE 1 150 g. of carbon black (#100 channel black, produced by Mitsubishi Kasei Co., Tokyo, Japan) heat treated previously at 1200 C. for 1 hour in vacuo, 66 g. of silicone varnish (KR 261 produced by Shinetsu Chemical Co., Tokyo, Japan) containing 50 wt. percent of a solid ingredient and 50 wt. percent of xylene as a solvent and 600 cc. of benzene are mixed together by using a ball mill of 21 cm. diameter. Said solid ingredient comprises methyphenyl-polysiloxanes containin gabout 24 wt. percent of Si. After the benzene and xylene are removed by heating the resultant mixture on a water bath, the mixture is heated at 220 C. for 2 hours to cure the silicone resin adhered to the surface of the carbon black. The cured mixture is pulverized for 1 hour with a ball mill of 21 cm. diameter. The pulverized material is fired in vacuo at 11000 C. for 2 hours so as to be converted into a high electric resistance material consisting of carbon particles coated, at least partially at the surface thereof, with the carbonized silicone resin. This resultant resistance material has the following typical properties:

(1) electric resistivity measured by using a cylinder of 8 mm. diameter made of phenol-formaldehyde resin, under pressure of 60 kg./cm. -0.327 9cm.

(2) bulk density measured by using a cylinder of 8 mm. diameter made of phenol-formaldehyde resin, under pressure of 60 kg./cm. -0.756 g./cm.

(3) Si-content in the resultant resistance material-3.38

wt. percent.

In a similar way to the process described above, one can obtain various high resistance materials which are different in the specific resistance and Si-contentshown in FIG. 1 and FIG. 2 by controlling the weight ratio of carbon black to silicone resin and carbonizing temperature.

Resistance elements are produced by mixing the resistance materials of the present invention, silica filler in an average particle size of 1.1 and epoxy resin in a weight ratio of 13167220 by use of a hot roller, extruding and molding the mixture to a shape of 3 mm. diameter and 10 mm. length, and finally aging the molded resistance elements at 200 C. for 4 hours. Said epoxy resin is prepared by mixing Epon 828 supplied by Shell Chemical Co. and 33 p.h.r. of curing agent, cliaminodiphenylsulfone.

The resistance values and the temperature dependences of resistance of resultant resistance elements are shown in FIG. 3 and FIG. 4.

Moreover, in a similar way to the process described above except for the mixing ratio of resistance material of the present invention, silica filler and epoxy resin, resistance elements of similar resistance value are produced. Weight ratio of the components, electrical properties of resultant resistance element and yields are shown in Tables 4 and 5. Yield means the percentage of numbers of resistance elements having resistance values falling within il0% of the average resistance value within the total number of 150 samples.

Referring to FIG. 1, three curves indicate a relation between a mixing ratio of silicone resin (solid ingredient in KR 261) to carbon black and Si-content of resultant resistance material. The variation in the Si-content with a change in the carbonizing temperature from 800 to 1200 C. is relatively small, because thermal decomposition of cured silicone resin is almost completely below 800 C.

1 Conventional sample employs resistance element comprising the same carbon black preheated at 1,200 O. for 1 hour in vacuo as used in Example 1.

TABLE 5 Conven- Sample No 1 2 3 tional 1 Resistance value (m9) 2. 32 2. 13 2. 2. 87 Voltage eoefllcicnt (percent/v.) 0. 014 0. 0016 0. 0022 0. 032 Noise factor (a v./v.) 0. 43 0. 49 0.62 l. 1 Yield (percent) 95. 7 98. O 91. 3 73. 3

Conventional sample employs resistance elements comprising the same carbon black preheated at 1,200 C. for 1 hour in vacuo as used in Example 1.

FIG. 2 indicates a relation between specific resistance and a carbonizing temperature of the resistance materials having various Si-contents. From FIG. 1 and FIG. 2, it will be apparent that the resistance materials having various specific resistance are obtained by selection of the mixing ratio of raw silicone resin to carbon black. FIG. 3 indicates a relation between resistance value of a resistance element comprising said resistance material and a carbonizing temperature of said resistance materials of various Si-contents. It will be apparent that resistance values necessary for practical use can be easily achieved by the resistance elements comprising the resistance materials according to the present invention. FIG. 4 indicates the temperature dependence of resistance of three resistance elements comprising the resistance materials carbonized at different temperatures. In FIG. 4 an ordinate axis expresses g X (percent) wherein R and R, are electric resistance values at 25 C. and t C., respectively.

A carbonizing temperature of resistance material has a great effect on the temperature dependence of resistance of resistance elements, for instance, a lower carbonizing temperature gives a greater temperature dependence of electric resistance below 25 C. It will be understood that resistance elements having desired temperature characteristics can be produced by selecting a suitable carbonizing temperature of the resistance material. The characteristics of the resistance elements comprising the resistance material according to the present invention are illustrated in Tables 4 and 5.

A resistance element in aforesaid size having an electric resistance of 2 to 3 M9 comprises about 7.5 wt. percent of carbon black having no surface layer coated with the carbonized silicone resin in accordance with the prior art. However, the resistance element defined above, for instance, can be prepared by employing about 9 to 13 wt. percent of resistance material having a surface layer at least partially coated with the carbonized silicone resin according to the invention. 9 to 13 Wt. percent of the resistance material can be calculated into 8.75 to 12.0 wt. percent of carbon content in the resultant resistance elements upon consideration of a Si-content contained in the resistance material. Therefore, the resistance materials according to the present invention can be used in a larger amount than the conventional carbon powder having no carbonized silicone resin. The higher weight percent of resistance material results in a stable and even composition and causes an increased yield and superior characteristics in the noise factor and voltage coefiicient as shown obviously in Table 5.

EXAMPLE 2 Resistance elements are prepared similarly to the process described Example 1 by employing the electric resistance material of this invention (B) and some conventional carbonaceous resistance materials (A) such as carbon black, heat treated carbon black, acetylene black and carbonized silicone resin.

Each of solid resistance elements defined by (A) and (B) consists of the resistance material defined above, silica filler of average particle size of 8,11 and epoxy resin (the same as used as Example 1) in a Weight proportion of 10:70:20.

Excelsior supplied by Columbian Carbon Co. is used as a conventional carbon black, and same carbon black heat-treated at 1200 C. for 1 hour in vacuo is used as heat-treated carbon black. Acetylene black supplied by Denki Kagaku Kogy, Tokyo, Japan, is used without any heat treatment. Carbonized silicone resin is made by curing the silicone varnish KR282 which is supplied by Shinetsu Chemical Co., Tokyo, Japan and which contains 50 wt. percent of solid ingredient (said solid ingredient comprises dimethyldiphenyl-polysiloxanes containing about 21 wt. percent of Si) and 50 wt. percent of xylene, carbonizing said cured silicone resin at 1000 C. for 1 hour in vacuo and pulverizing said carbonized silicone resin under 200 meshes. The electric resistance material of this invention is prepared by the process as follows.

50 g. of Excelsior carbon black, 30 g. of Excelsior carbon black preheated at 1200 C. for 1 hour in vacuo, 100 g. of silicone varnish (KR 282) and 200 cc. of trichloroethylene are mixed together by a ball mill of 12 cm. diameter. After the evaporation of xylene and trichloroethylene, the mixture is cured at 200 C. for 2 hours. The cured mixture is pulverized for hours with a ball mill of 12 cm. diameter. The resultant pulverized material is then fired at 1000 C. for 2 hours in vacuo so as to make the electric resistance material of this invention.

Table 6 shows the electric resistance values and temperature dependences of the resistance elements employing the resistance materials described above.

TABLE 6 Temperature dependence of resistance Rr-Rrs (percent) R25 Resistance material Resistance 55 C. C. employed value (t:55) (t=+l05) 1. Carbon black 5.0 mil +34 9 2. Heat-treated carbon black 2. 5 k9 +7 +15 3. Acetylene black U. 5 kn +4 +4 4. Carbonized silicone resin 800 mi: +53 13 (B) The present; invention 3.5 mi) +12 +7 Standard MIL-R-11C 1. 1-10 k9 =|=10 5:6 1. l-l0 mil $20 5:15

It is apparent from Table 5, the resistance element comprising the A1, i.e. carbon black has temperature dependence of resistance differing greatly from the Standard, particularly at 55 C. The resistance element comprising A-2, i.e. heat-treated carbon black has an extremely low electric resistance and a great temperature dependence of resistance at 105 C. The resistance element comprising A-3 or A-4, i.e. acethylene blac or carbonized silicon resin is not also satisfactory. The resistance element comprising the novel resistance material, B according to the present invention is provided with a relatively high resistance and a satisfactory temperature dependence of resistance and moreover has a superior noise factor, voltage coefficient of resistance and the production yield similar to those of Example 1.

What we claim is:

1. A method for making a resistance material comprising mixing carbonaceous minute particles and a solvent solution of thermosetting silicone resin selected from the group consisting of alkyl-silicone, aryl-silicone and alkyl-aryl-silicone, evaporating the solvent in said solution, heating the resultant mixture to cure said silicone resin, pulverizing the heated mixture and firing said pulverized material to carbonize the cured silicone resin applied to, at least partially, the surfaces of said carbonaceous minute particles at a temperature from 700 to 1300 C. in a non-oxydizing atmosphere.

2. A method for making a resistance material defined by the claim 1, wherein a weight ratio of said thermosetting silicone resin to said carbonaceous minute particles is from 0.02 to 1.

3. A method for making a resistance material defined by the claim 1, wherein said carbonaceous minute particles are heated in a non-oxidizing atmosphere at a temperature of 700 to 2500 C. prior to mixing with the silicone resin.

4. A method for making a resistance material defined by the claim 1, wherein said carbonaceous minute particles are a member selected from the group consisting of channel black, furnace black, thermal black, acetylene black and lamp black.

5. A method for making a resistance material defined by the claim 1, wherein said thermosetting silicone resin is a member selected from the group consisting of methylsilicone and ethyl-silicone.

6. A method for making a resistance material defined by the claim 1, wherein said thermosetting silicone resin is a member selected from the group consisting of methylphenyl-silicone and ethyl-phenyl-silicone.

7. A method for making a resistance material defined by the claim 1, wherein said thermosetting silicone resin is modified with an organic plastics selected from the group consisting of alkyd, phenol, polyester, melamine and epoxy resin.

8. A method for making a resistance material defined by the claim 1, wherein the carbonizing temperature is in the range from 700 to 1000 C.

9. A method for making a resistance element which comprises mixing the resistance material according to the claim 1 and a polymerizable resin, forming the mixture into a given shape and curing said polymerizable resin.

9 10 10. A method for making a resistance element defined 2,891,879 6/ 1959 Rohrer 2525 11 by the claim 9, wherein said polymerizable resin is 21 3,056,750 10/ 1962 Pass 252-511 thermosetting resin.

DOUGLAS J. DRUMMOND, Primary Examiner References Cited 5 UNITED STATES PATENTS US. Cl. X.]R. 2,559,077 7/1951 Johnson 252--511 117.42 2 4 29 

